With radiation therapy, radiological data is acquired, for example with the aid of CT scans, for planning the irradiation of a patient in order to be able to define the radiation dose for the planned irradiation. In particular, it is important to define radiation doses in a spatially resolved manner in order to destroy only malignant tissue in the region to be irradiated and to spare neighboring, potentially very sensitive regions in the body of the patient.
The interactions between radiation and tissue that occur during irradiation of the patient can be divided into primary and secondary effects. The primary effects are the direct interaction of the radiation with the tissue. In the case of irradiation with photons, the interaction primarily occurs with electrons. If tissue with heavy particles is irradiated, then the interaction primarily occurs with the atomic nuclei. In addition, in the case of the described primary processes, so much energy is transferred to the electrons during the interaction that they are released from the molecule and have enough energy themselves to cause further ionization processes as a secondary effect. Different effects occur when electromagnetic radiation interacts with electrons. In the case of absorption of radiation in soft tissue, which is primarily composed of water, the Compton effect dominates; in the case of absorption in solid body substance, such as, for example, bone substance, the photo effect dominates.
To be able to determine the radiation dose in advance, the charge carrier density distribution, i.e. in particular the electron density distribution, or the nuclear charge carrier density distribution of the materials present in the region to be examined must be known.
A conventional method for determining electron densities using CT image data sets consists in mapping attenuation values of the CT image data, hereinafter also called CT values for short, on electron densities with the aid of a simple table. However, this method does not achieve a very high level of accuracy because in the case of the polychromatic X-ray radiation used in CT scans, CT values of the same material in the image are dependent on the size of the object to be examined in which they are scanned, and are also dependent on the position in the cross-section of the object. This is due to the fact that, owing to the increased radiation hardness, a near-surface volume element is exposed to a softer radiation during imaging than a centrally located volume element. With the same density and material, a higher CT value (stronger attenuation) is therefore associated with the near-surface volume element than with the centrally located volume element. Owing to the different CT values, a higher electron density is therefore associated with the near-surface volume element than the centrally located volume element. The accuracy of this method is therefore also limited if a calibration has previously been carried out with the aid of a test body (what is known as a phantom) in a very accurate and reproducible manner.
Another way of determining charge carrier densities is based on the CT scan with the aid of two spectra, also called dual-energy CT, wherein the recorded scan data is depicted in a base material breakdown. The scan data divided according to individual materials can then be mapped again on charge carrier densities. As already mentioned, the absorption properties of the biologically relevant materials are essentially based on just two different effects, the photo effect and the Compton effect, so a breakdown of the scan data according to two base materials, for example water or soft tissue and calcium, is sufficient. In this way, the effect of the patient's size and the position of a volume element in the body of the patient is reduced for these materials.
However, not every CT device has the option of a dual-energy scan, so this method is only available to a limited extent.